linux-kernel - Re: [v5 2/4] mm, oom: cgroup-aware OOM killer

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Date:   Wed, 23 Aug 2017 17:20:31 +0100
From:   Roman Gushchin <guro@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, <kernel-team@...com>,
        <cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [v5 2/4] mm, oom: cgroup-aware OOM killer

Hi Johannes!

Thank you for review!

I do agree with most of the comments, and I will address them in v6.
I'll post it soon.

Please, find some comments below.

On Tue, Aug 22, 2017 at 01:03:44PM -0400, Johannes Weiner wrote:
> Hi Roman,
> 
> great work! This looks mostly good to me now. Below are some nitpicks
> concerning naming and code layout, but nothing major.
> 
> > +
> > +	css_task_iter_start(&memcg->css, 0, &it);
> > +	while ((task = css_task_iter_next(&it))) {
> > +		/*
> > +		 * If there are no tasks, or all tasks have oom_score_adj set
> > +		 * to OOM_SCORE_ADJ_MIN and oom_kill_all_tasks is not set,
> > +		 * don't select this memory cgroup.
> > +		 */
> > +		if (!elegible &&
> > +		    (memcg->oom_kill_all_tasks ||
> > +		     task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN))
> > +			elegible = 1;
> 
> This is a little awkward to read. How about something like this:
> 
> 	/*
> 	 * When killing individual tasks, we respect OOM score adjustments:
> 	 * at least one task in the group needs to be killable for the group
> 	 * to be oomable.
> 	 *
> 	 * Also check that previous OOM kills have finished, and abort if
> 	 * there are any pending OOM victims.
> 	 */
> 	oomable = memcg->oom_kill_all_tasks;
> 	while ((task = css_task_iter_next(&it))) {
> 		if (!oomable && task->signal_oom_score_adj != OOM_SCORE_ADJ_MIN)
> 			oomable = 1;
> 
> > +		if (tsk_is_oom_victim(task) &&
> > +		    !test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags)) {
> > +			elegible = -1;
> > +			break;
> > +		}
> > +	}
> > +	css_task_iter_end(&it);

We ignore oom_score_adj if oom_kill_all_tasks is set, it's
not reflected in your version. Anyway, I've moved the comments block
outside and rephrased it to make more clear.

> 
> etc.
> 
> > +
> > +	return elegible > 0 ? memcg_oom_badness(memcg, nodemask) : elegible;
> 
> I find these much easier to read if broken up, even if it's more LOC:
> 
> 	if (eligible <= 0)
> 		return eligible;
> 
> 	return memcg_oom_badness(memcg, nodemask);
> 
> > +static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> > +{
> > +	struct mem_cgroup *iter, *parent;
> > +
> > +	for_each_mem_cgroup_tree(iter, root) {
> > +		if (memcg_has_children(iter)) {
> > +			iter->oom_score = 0;
> > +			continue;
> > +		}
> > +
> > +		iter->oom_score = oom_evaluate_memcg(iter, oc->nodemask);
> > +		if (iter->oom_score == -1) {
> 
> Please add comments to document the special returns. Maybe #defines
> would be clearer, too.
> 
> > +			oc->chosen_memcg = (void *)-1UL;
> > +			mem_cgroup_iter_break(root, iter);
> > +			return;
> > +		}
> > +
> > +		if (!iter->oom_score)
> > +			continue;
> 
> Same here.
> 
> Maybe a switch would be suitable to handle the abort/no-score cases. 

Not sure about switch/defines, but I've added several comment blocks
to describe possible return values, as well as their handling.
Hope, it will be enough.

> >  static int memory_events_show(struct seq_file *m, void *v)
> >  {
> >  	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> > @@ -5310,6 +5512,12 @@ static struct cftype memory_files[] = {
> >  		.write = memory_max_write,
> >  	},
> >  	{
> > +		.name = "oom_kill_all_tasks",
> > +		.flags = CFTYPE_NOT_ON_ROOT,
> > +		.seq_show = memory_oom_kill_all_tasks_show,
> > +		.write = memory_oom_kill_all_tasks_write,
> > +	},
> 
> This name is quite a mouthful and reminiscent of the awkward v1
> interface names. It doesn't really go well with the v2 names.
> 
> How about memory.oom_group?

I'd prefer to have something more obvious. I've renamed
memory.oom_kill_all_tasks to memory.oom_kill_all, which was earlier suggested
by Vladimir. Are you ok with it?

Thanks!